1. Field of the Invention
This invention relates to injection molding systems.
More specifically, the present invention relates to a valve gating system found in injection molding systems.
2. Summary of the Prior Art
Injection molding nozzles are well known and are used to inject materials, such as plastic, into cavities of a mold. For example, such nozzles receive molten material, such as plastic, from an injection molding machine and direct the same into mold cavities through passages called gates. When an injection operation is complete, and prior to opening the mold cavity to eject the molded parts, the transfer of molten material through the gates must be stopped. Generally, two methods exist for stopping the transfer of molten material through the gates, namely: thermal, or open, gating; and valve gating.
In thermal gating, the gate is an open aperture through which molten material passes during an injection operation. The gate is rapidly cooled at the end of the injection portion of the cycle, when the injection pressure is removed, to “freeze” the injected material into a plug. This plug remains in the gate to prevent drool of molten material from the gate when the mold is open for the ejection of the molded part. In the next injection portion of the cycle, the cooling applied to the gate is effectively removed and hot molten material from the injection molding machine pushes the remaining plug into the mold cavity, where it melts and mixes with the newly provided molten material.
In valve gating, the opening and closing of the gate is independent of injection pressure and/or cooling and is achieved mechanically with a valve stem. This stem can be moved between an open position, wherein flow of molten materials through the gate is permitted, and a closed position wherein the gate is closed by entry of the valve stem into the gate which establishes a seal, preventing molten materials from passing through the gate. Valve gating is well known and examples of such systems are shown in U.S. Pat. Nos. 2,878,515; 3,023,458; and 3,530,539, each being incorporated herein by reference.
Generally, for situations that require improved aesthetics, valve gating is preferable to thermal gating because it can reduce the undesired gate vestige which results on the finished molded part. However, there are problems with valve gating systems.
Specifically, the valve stem and gate each have mating sealing portions with a typical diametrical clearance of 0.001 to 0.002 inch between the valve stem and the gate sealing portions. As the valve stem is moved into alignment with the sealing portion of the gate to effect sealing, a slight misalignment of the stem with the gate will cause the stem to strike the gate sealing portion. Over time, this will cause the gate area to wear and become misshapen. Once the gate sealing area is worn, the stem no longer stops the flow of molten material and a small amount of molten material will migrate between the stem and the worn gate sealing area. This leakage adversely impacts the vestige quality because as the mold is opened, the now-solidified material between the gate and the valve stem will cause a tear or blemish to form along the vestige of the part, and in extreme cases, the tearing can propagate to the surface of the molded article or preform.
Following the injection cycle, typically the mold halves will open and the molded article in a somewhat solidified state will be removed from the stem/gate area. Due to the entrapped molten material between the worn gate area and the stem, the molded article will not break away cleanly when the mold is opened, but rather will tear away from the gate area, which results in a blemished vestige on the molded article.
Referring to FIGS. 1 and 2 this phenomenon can be related. As well known in the art, a nozzle assembly 10 is comprised of an elongated nozzle bushing 12 with a nozzle tip 16 affixed co-axially therein. Optionally, an insulator 14 is affixed to a proximal end of the nozzle tip 16 thereby thermally insulating the heated nozzle assembly 10 from a cooled gate insert 31 and cavity insert 34. A movable valve stem 18 extends co-axially in the nozzle assembly 10 and is selectably positioned in or out of a passageway 22 in the nozzle. A melt channel 20 surrounds the valve stem 18 and runs the length of the nozzle assembly 10 to communicate a molten material to a mold cavity 28. When the valve stem 18 is placed in a fully closed position (as shown in FIG. 1), valve stem 18 extends into a gate sealing portion 25 in the gate insert 31. The sealing portion 25 sealingly surrounds the valve stem 18 to shut off the flow of material to the mold cavity 28. As shown in FIG. 1, a face portion 21 of valve stem 18 and a gate vestige forming portion 35 of the gate insert 31 define the entire top and side of the molded article vestige 26. A chamfer 36 is typically provided along the face of the valve stem 18 to help guide the valve stem into the sealing portion 25 of gate insert 31 and reduce wear of the valve stem and gate inset 31.
Due to the close fit of the valve stem 18 to the sealing portion 25, any misalignment that exists between their respective interfaces will cause the valve stem 18 to strike the surface of the sealing portion 25 which will ultimately lead to a deterioration of the sealing portion 25 and/or the valve stem 18. Gate insert 31 provides a component that can be replaced as the sealing portion 25 wears rather than replacing the entire cavity insert or plate if the sealing portion 25 and the recess 11 for receiving nozzle assembly 10 were formed directly therein, as may still be done with smaller number of cavities. However, gate insert 31 still is a fairly detailed component and it is undesirable to replace it unless absolutely necessary.
At the end of the injection cycle, the valve stem 18 is moved into its closed position as previously described and the molding inserts, including core 30, are held in a closed position for a predetermined cycle time to allow the molten material to cool and solidify, thereby forming the molded article. Once the molded article has been allowed to cool to a sufficient level, the core 30 with the molded article thereon is moved away form the gate insert 31 and the vestige 26 is pulled away from the face portion 21 of the valve stem 18. If enough wear exists between the valve stem 18 and the sealing portion 25, a small amount of molten material will have migrated therein, and consequently as the vestige 26 is moved away form the vestige forming portion 25 a peeled edge 38 will form on the vestige 26 of the molded article 27.
Also, since the valve stem 18 is surrounded by molten material, it becomes quite hot. When the gate is closed by the valve stem 18, the hot tip of the valve stem 18 cools slower than the gate insert 31 as the mold cavity 28 is cooled. Ideally, molded article 27 is not removed from the mold until the vestige 26 has cooled sufficiently to allow a clean separation of the solidified material at the face portion 21 of the valve stem 18. With the valve stem being hot compared to the gate insert, this can require increased cycle times to permit the necessary cooling and/or can result in undesirable characteristics in the molded article 27. Specifically, as the material in the mold cavity 28 adjacent the valve stem 18 is cooled relatively slowly due to the hot valve stem 18, parts molded from thermally sensitive materials, such as PET, can suffer from an enlarged area of crystallinity 40 or other undesired characteristics. To reduce cycle times, a mold may be opened before the material adjacent the face portion 21 has sufficiently solidified. Since the entire top surface of the vestige 26 is in contact with the face portion 21 of the hot valve stem 18, stringing and an uneven edge may form when the mold is opened.
Therefore, there is a need for an improved valve gate system that reduces or obviates some or all of the drawbacks of the prior art.